15.3: Kelp Forests

The name kelp is given to a number of species of brown macroalgae that are attached to the seafloor at depths as great as 30 to 40 m, where the water is clear enough to allow light to penetrate to that depth. Once established, kelp fronds grow toward and eventually reach the ocean surface. Where conditions are suitable, kelp grows so densely that it forms forests (Fig. 15-8).

 

Figure 15-8. Kelp forest. (a) Kelp, such as this Macrocystis pyrifera, is attached to the seafloor by holdfasts and has long, flexible stems, called “stipes,” that support the fronds in which photosynthesis occurs. (b) In some areas of abundant kelp, the ocean surface is covered by a canopy of kelp fronds, and the kelp growing upward from the seafloor forms an underwater forest. (c) The sunlight filters through the canopy and kelp forest in this underwater photograph from about 8 to 10 m depth in Monterey, California.

Extensive forests of one such kelp, Macrocystis pyrifera, are present off the west coast of North America. Kelp fronds are buoyant. Macrocystis pyrifera fronds, for example, have gas-containing sacs or bladders distributed along them (Fig. 12-4a). Once a frond reaches the surface, it continues to grow, but the new growth floats on the surface to form dense mats or a canopy. These canopies can cover the entire ocean surface within the species’ depth range along substantial stretches of coast.

Kelp Community Environmental Characteristics

Kelp forests grow where the temperature and nutrient characteristics of the water column are very different from those required for coral reef formation. Kelp requires water cooler than 20°C and high nutrient concentrations. Although kelp is attached to the seafloor by holdfasts, it obtains its nutrients from the water column through the surface of its fronds. Kelp fronds grow as much as half a meter a day, and kelp forest primary productivity ranges from 500 to 1500 g of carbon per square meter per year. This rate exceeds the primary productivity in all but the most highly productive phytoplankton-based ecosystems and is approximately the same as primary production rates of terrestrial farms. Large quantities of nutrients are necessary to sustain such growth rates. Consequently, kelp forests are almost exclusively restricted to areas of intense upwelling. The fact that upwelled waters are cold probably accounts for kelp’s temperature niche requirements.

Besides low temperatures and high nutrient concentrations, kelp needs a stable, generally rocky, seafloor to which it can attach. In addition, kelp can grow only in shallow waters where sufficient light penetrates to the seafloor to support the growth of newly settled kelp spores.

Kelp Life Cycle and Communities 

The kelp life cycle consists of two distinctly different stages. Although the large kelp that form kelp forests grow vegetatively (asexually), they also produce microscopic swimming spores that enter the plankton and eventually settle on the seafloor. These spores grow into microscopic male and female plants, which produce eggs and sperm and reproduce sexually near or on the seafloor. If they encounter suitable conditions, the resulting embryos then grow into the large, mature kelp. 

Kelp forests support amazingly diverse populations of invertebrates, fishes, and marine mammals. The kelp provides several benefits to species that live within its forests. First, kelp produces large quantities of detritus, which forms the base of the food web. Surprisingly, only a few animals, including only a few fish, snail, and sea urchin species, eat kelp itself, but kelp is easily torn apart, particularly at the ends where new growth occurs. Therefore, kelp releases large amounts of detritus, which is first modified by decomposers and only then consumed by animals. Second, the kelp canopy provides a hiding place and protection from predators, particularly seabirds. The fronds afford many escape routes that enable agile harbor seals and sea lions to evade their shark and killer whale predators, which are less agile and less able to maneuver through the kelp forest. In addition, kelp fronds and holdfasts provide substrate for encrusting organisms and their grazers or predators and many secluded places for small fishes and invertebrates to hide from predators.

The rocky seafloor and holdfasts of the kelp forest also sustain a surprisingly diverse community of nonhermatypic corals, anemones, crabs, shrimp, sea stars, nudibranchs, fishes, and other animals (Fig. 15-9). In many ways, kelp forest communities rival those of coral reefs in complexity and beauty.

 

 

 

 

Figure 15-9. Members of the kelp holdfast community. (a) These small brown cup corals (Paracyathus stearnsii, California) cover many of the rocks at shallow depths. (b) Anemones, such as this rose anemone (Tealia lineata, California), are abundant on the kelp forest floor. (c) Many species of crabs, such as this northern kelp crab (Pugettia producta, California), feed on the abundant detritus and other animal life. (d) A kelp shrimp is camouflaged to look like the algae in which it lives and on which it feeds (Monterey, California). (e) Sea stars of many species live on the kelp forest floor, and sea pens are often found in the patchy areas of seafloor that are located in and around the forest (Monterey, California). (f) This horned nudibranch species (Hermissenda crassicornis, California) is common in the kelp community, together with many other nudibranch species. (g) Many rockfish species live and hunt in the kelp forest, such as these two: the quill-backed rockfish (Sebastes maliger) on the lower left, and the copper rockfish (Sebastes caurinus, California) swimming above, further to the right.

Kelp, Sea Otters, and Sea Urchins

Two of the most important residents of the kelp community are sea otters (Fig. 15-10a) and sea urchins (Fig. 15-10b). Sea urchins eat kelp, and sea otters eat sea urchins. Because of this relationship, healthy and abundant kelp forests depend to a large extent on a healthy population of sea otters. In areas where otters are abundant and many sea urchins are eaten, sea urchin populations are low and dominated by small, young individuals. Such populations are too small to affect the kelp significantly (Fig. 15-10c). However, if otters are scarce, the sea urchin population multiplies, and many large sea urchins are present to graze heavily on the kelp and reduce its abundance (Fig. 15-10d). Large sea urchins, in particular, feed preferentially on the kelp holdfasts and may destroy the forest by cutting the kelp loose from the seafloor, even if they do not consume it.

 

 

Figure 15-10. The sea otter–urchin–kelp relationship. (a) Sea otters (Enhydra lutris) were at one time abundant in the kelp forests of California, but their numbers plummeted during the nineteenth century because they were exploited for their pelts. Their populations are now recovering. (b) The sea otter’s favorite and most important food is sea urchins, such as this red urchin (Strongylocentrotus sp., California). (c) At Amchitka Island, Alaska, where sea otters are abundant, sea urchins are rare in shallow water because they are preyed upon by the otters. As a result, there is little browsing of the kelp forests by urchins and the kelp is abundant. (d) In contrast, at nearby Shemya Island, where otters have become scarce, urchins are abundant and kelp is very sparse as a result of browsing by the urchins.

The California coast was once almost completely fringed by kelp forests, but they were severely depleted by the early years of the twentieth century and have only recently begun to recover. The primary reason for the kelp forest decline is now understood to be hunting of sea otters during the eighteenth and nineteenth centuries, which drove the otters almost to extinction. Once the otters were removed, sea urchin populations increased and steadily overwhelmed and destroyed the kelp forests. Now sea otters are protected and their populations are slowly recovering. As a result, sea urchin populations are declining and kelp forests are gradually returning, although their future is threatened by climate change-driven warming of ocean waters. The sea otter is called a “keystone predator” because, without it to prey on urchins, the urchin population growth produces large and fundamental changes in the ecosystem. Keystone predators are found in many ecosystems.